Pierre Charles Alexandre Louis (1787–1872) represented one side in a three way argument about the source of medical knowledge and reasoning which continues today.

Knowledge can come from the physician’s long astute clinical experience. This approach goes back to the ancient Greek school of medical empirics 280 BC. The “tripod of the empirics” was to learn from one’s own chance observations, to learn from your colleagues present and past, and to reason by analogy (“this patient looks like one I saw before”).¹

In the Paris of Louis’s day the second approach to medical knowledge—experimental physiology—was being developed by Xavier Bichat, Claude Bernard, Francois Magendie, and others. They used theory and hypothesis testing on controlled animal experiments^2,3 to show the cause of disease. Today this kind of reasoning might go as follows. Variation in DNA at this location causes these proteins to malfunction causing changes at the cell level which modify organ systems leading to symptoms and disease. This is a logical causal sequence, built on theory, and tested by controlled laboratory experimentation.

Louis brought a third approach—his mathematical method. He carefully recorded for many patients their characteristics, symptoms, treatment and outcomes and he related symptoms and treatments to outcomes. His best known study was the evaluation of blood letting as a treatment.

BLOOD LETTING

Blood letting was standard practice in medicine for over 2500 years and reached its zenith in the mid 1800s. It was performed either by cutting a vein (or, less commonly, an artery) with a lancet, by using leeches to suck blood from a localized area, or by “cupping”, a procedure to draw the blood to the localized skin surface. In just one year (1833) it is estimated that France imported 42 million leeches,⁴ and it has been estimated that up to five million litres …